Ohio today became the first state in America to put to death a prisoner using a single drug lethal injection in a technique that lawyers and campaigners have criticised as human experimentation.

Kenneth Biros was pronounced dead at 11.47am today, about 10 minutes after being given an overdose of the powerful anaesthetic thiopental sodium through an intravenous drip (IV) into his left arm.

The procedure was introduced by the Ohio state authorities to circumvent legal challenges brought after Romell Broom was given a stay of execution as he was actually lying on the gurney after technicians had tried and failed for two hours to give him the three-drug lethal injection - the most common form of capital punishment in America. He was sent back to death row. The executioners had been unable to find a working vein to insert the IV. The new single-drug method was designed to allow for a back-up position in which a combination of two painkillers could be injected directly into muscle.

Paradoxically, the executioners again struggled for up to half an hour on Tuesday to find a vein in Biros in which to put the IV through which his single anaesthetic was administered. His lawyers, who had made numerous attempts to persuade the courts to postpone his execution on grounds that the technique was untried and amounted to human experimentation, said the procedure had proven flawed.

John Parker, one of Biros' lawyers, said he had counted nine attempts to find a vein in the prisoner's left arm.

Deborah Denno, a specialist in execution methods at Fordham University in New York, said on the one hand it was good news that Ohio had dropped the use of a paralytic agent - the second in the three-drug cocktail used by all 35 other death penalty states - because that had been shown to induce extreme pain, but finding veins was still clearly a problem, and the so-called back-up of injecting painkillers into muscle was untested and could lead to a lingering death. She predicted future challenges.

Debi Heiss, sister of Tami Engstrom who was murdered by Biros in 1991 when she was 22, attended the execution, and told the Columbus Dispatch that it had gone "too smooth. I think he should have gone through some pain for what he did".

Ohio will today reconsider whether to send Broom back to the death chamber under the single-drug policy.